The goal is to develop a clinical prototype of the Laser Optoacoustic Imaging System (LOIS), a new concept in diagnostic imaging of breast cancer. Specific aims focus on (1) tests of LOIS using phantoms resembling optical and mechanical properties of breast with tumors, (2) the system improvement on the basis of results of initial tests, (3) evaluation of the system using mastectomy specimens, (4) modification of the system in preparations for the in vivo clinical trials and finally, (5) clinical trials on 50-100 patients. In vivo optical tomography studies suggest that enhanced light absorption in breast cancer compared with normal breast tissues can help to reveal malignant tumors on the basis of optical contrast. The hypothesis is that LOIS utilizing sensitive detection of pressure waves preferentially induced by laser pulses in tumors will provide breast cancer images with greater contrast and sensitivity compared with x-ray mammography, ultrasonography or optical tomography. The major idea is that energy of short laser pulses absorbed primarily in tumors will create sources of pressure waves in ultrasonic frequency range. The ultrasonic waves will deliver diagnostic information to the breast surface, where signals can be detected by piezoelectric transducers. Reconstruction of a three-dimensional image of breast will be obtained on the basis of time-resolved detection and analysis of profiles of laser-induced pressure distribution recorded with a number of transducers located along the breast surface. Modalities presently applied for breast cancer detection are not free of serious limitations. The applicants propose to combine advantages of optical and ultrasonic imaging in one advanced tomography.